1. Technical Field
This invention relates to cordless digitizers and, more particularly, to a device for imparting a signal phase status of a signal transmitting from the cursor.
2. Background Art
Digitizer systems for use as input devices to computers are well known in the art. In a typical digitizer system, a cursor is moved over the working surface of a tablet wherein the working surface of the tablet defines the boundaries of an X-Y coordinate system. Sometimes, the position of the cursor on the working surface of the tablet is determined by electrostatic means. For many reasons, an electro-magnetic sensing link between the cursor and the tablet provides superior results and is preferred.
Recently, so-called "cordless digitizers" in which there is no connecting cable between the cursor and the tablet have become popular. In an electro-magnetic version as manufactured by the assignee of this application and as depicted in simplified form in FIG. 1, the digitizer tablet 10 employs grid conductors 12 for each of the two coordinate directions (only one being depicted for simplicity). The cursor 14 is the "driven" member and emits an alternating current (AC) magnetic field from its tip at a given frequency. The magnetic field emanations induce signals into the grid wires 12 which are then used by the tablet electronics and logic (not shown) to determine the location of the cursor 14. In practice, first the grid conductors 12 for one coordinate direction are individually selected by a selector device 15 and the induced signal therein is sensed. Then the grid conductors 12 for the other coordinate direction are similarly selected and sensed. The amplitude characteristics of the induced signal and its magnitude are used by the tablet electronics and logic to determine how close the cursor is to the grid conductor being sensed. When all the grid conductors have been sampled, the location of the cursor on the tablet's working surface is derived.
In an implementation such as shown in FIG. 2, where any given grid conductor 12 crosses the working surface area only once, an amplitude characteristic associated with the signal induced in the grid conductor by a cursor positioned in the middle of the group of conductors 12 is depicted in FIG. 3. It should be noted that the cursor location is indicated by the null in the plot of the amplitude characteristic.
In a case where a large sensing area is needed, it is often desired that the number of grid conductors 12 be kept at a minimum to simplify construction of the tablet 10 and reduce the manufacturing costs. This is accomplished by having an individual conductor cross the working surface more than once. FIG. 4 shows an implementation of a digitizer where each conductor 12 crosses the working surface of the tablet 10 twice. This configuration results in one leg of a conductor 12 being in one half of the tablet 10 and the other leg being in the other half of the tablet 10. However, an ambiguity as to the position of the cursor 14 arises in such a configuration. The ambiguity results because a cursor 14 placed near one side of a first leg of a conductor 12, will induce a signal in the conductor 12 as viewed from the selector 15 having the same voltage magnitude as the signal induced if the cursor 14 had been placed the same distance from the other leg of the conductor 12. For example, a cursor 14 placed at location "A" in FIG. 4 would induce a signal having an identical voltage magnitude as that induced had the cursor been placed at location "B". Therefore, the aforementioned ambiguity as to the position of the cursor 14 must be resolved in a digitizing system employing conductors 12 which cross the working surface of the tablet 10 more than once.
The ambiguity can be resolved if the phase of the signal induced in the conductors 12 is known relative to the cursor signal. FIG. 5 depicts the amplitude/phase characteristics that would result from a cursor 14 placed at positions "A" and "B" of FIG. 4. As can be seen the phase of the signal induced at location "A" is opposite of the signal induced at location "B". The cursor signal will be in phase with the induced signal at one of the locations and 180 degrees out of phase at the other location. Which location corresponds to which phase is simply a matter of convention and can be made to be either way. Accordingly, by knowing the phase of the cursor signal, the actual location of the cursor 14 can be differentiated from the anomalous location.
In past implementations, a wired connection between the cursor and the tablet was used to determine the phase of the field transmitting from the cursor. However, in a cordless cursor system, alternate means must be employed to acquire this phase information.
According to some prior art techniques, additional pickup conductor loops are incorporated in addition to the necessary position sensing conductors. However, these extra loops require additional conductors on the sensing grid. These extra conductors increase undesirable grid capacitance and make manufacturing the grid more difficult and costly. This technique also requires that the grid employed in a cordless cursor system to be different from that in a corded system. Therefore, a system capable of determining the phase of the cursor signal in a cordless digitizer without the addition of extra conductors is needed.
In another prior art approach invented by the assignee of this application, for which a now co-pending patent application was recently filed, the phase is determined by employing a large first pulse from the cursor. The problem is that under normal circumstances, the cursor emits its signals in bursts. While the phase of the emitted burst is always in the same direction, the tablet electronics and sensing circuitry cannot be sure that the first signal sensed is, in fact, the first signal emitted because the ringing oscillator circuit in the cursor that causes the cursor to emit its magnetic field signal burst starting at a low level and building to its full signal strength. According to the technique of that invention, the ringing oscillator circuit is stimulated with a large initial power surge which causes the first (i.e. known phase) output to be at a guaranteed sensible level. Thus, the tablet knows that the first waveform of the sensed signal is the first emitted waveform of known phase and the phase of the remaining signal can be determined therefrom.
While the large first pulse approach of that invention appears to work well for its intended purpose, there is some concern that under certain circumstances and configurations, as associated with pen-driven computing combined display and input devices in particular, the large first pulse approach may be susceptible to noise problems.
Wherefore, it is the object of this invention to provide an alternate approach for determining the phase of the magnetic field transmitting from a cursor in a cordless digitizer system employing conductors which cross the working surface more than once.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.